1. Field of the Invention
The present invention generally relates to power feed path switching circuits, and more particularly to a power feed path switching circuit which is provided in a branching unit for branching communications paths in a communications system.
An optical underwater communications system uses an underwater branching unit for branching an optical fiber cable in order to connect a plurality of points (stations). Normally, such an underwater branching unit has the function of switching power feed paths in order to prevent communications from being broken due to a fault in a power feed path. Recently, the number of underwater branching units used in a single communications system has been on the increasing trend. Hence, it is desired to provide a power feed path switching circuit capable of switching power feed paths with ease.
FIGS. 1A and 1B are block diagrams illustrating conventional power feed switching. Referring to FIG. 1A, a branching unit (BU) 10 connects a cable extending from a station A and a cable extending from a station B in series. A repeater (not shown in FIG. 1A), which is provided in the cables, is supplied with power from both the stations A and B in the two directions. The above power feed is called two-end power feed. If a fault has occurred in one of the two stations A and B, the repeater can be supplied with power from the other station. Hence, the two-end power feed shown in FIG. 1A has high reliability. A cable extending from a station C is ground to the sea SE (sea earth) in the branching unit 10. A repeater provided in the cable extending from the station C is supplied with power from only the station C. This power feed is called single-end power feed.
If a fault has occurred in the cable connecting the branching unit 10 and the station B, the cable extending from the station B is grounded to the sea in the branching unit 10, as shown in FIG. 1B. Further, the branching unit 10 connects the station A to the station C in series.
FIG. 2 shows a conventional power feed path switching circuit provided in the branching unit 10 shown in FIGS. A and 1B. An end 21 of the cable connected to he station A is connected to an end of a relay coil RL2 via a relay switch rl3. Similarly, ends 22 and 23 of the cables connected to the stations B and C are connected to ends of relay coils RL1 and RL3 via relay switches rl2 and rl1, respectively. The other ends of the relay coils RL1, RL2, and RL3 are connected to each other at a branching node X.
When there are no currents flowing in the relay coils RL1, RL2 and RL3, the relay switches rl1, rl2 and rl3 connect respective terminals a and b. When predetermined amounts of currents flow in the relay coils RL1, RL2 and RL3, the relay switches rl1, rl2 and rl3 connect terminals a and c, and the cable ends 23, 22 and 21 are grounded to the sea (SE), respectively.
In order to perform the two-end power feed between the stations A and B and perform the single-end power feed between the station C and the sea ground SE, a constant current I is made to flow in the relay coil RL1 from the station B, and a constant voltage is applied to the cable at the station A. In this case, the constant current I has a large amount enough to drive the relay switch rl1, and the constant voltage has a value which causes the potential of the node X to be set equal to the ground level. In response to the constant current flowing in the relay coil RL1, the relay switch rl1 connects the terminals a and c. Since the potential of the node X is equal to the ground level when the switch rl1 operates, a hot switching phenomenon can be prevented in which a charge stored in the cable between the station C and the branching unit 10 flows to the sea ground via the terminals a and c. If such a charge flows, the terminals a and c may be damaged.
Recently, there has been an increasing trend to use a large number of underwater branching units used in a single communications system. FIG. 3 shows a communications system having n branching units (BU) 31.sub.1 -31.sub.n where n is an integer. The n branching units 31.sub.1 -30.sub.n are cascaded, and terminal stations 30.sub.0 -30.sub.n+1 are connected to these branching units, as shown in FIG. 3.
A description will now be given of power feed between the terminal stations 30.sub.0 -30.sub.n+1. It will now be assumed that currents necessary to switch the relay switches rl1 of the branching units 31.sub.1 -31.sub.n are denoted by I.sub.1 -I.sub.n in which I.sub.1 &lt;I.sub.2 &lt;. . . &lt;I.sub.n.
FIGS. 4A, 4B and 4C illustrate a procedure for performing a switching operation on the branching units 31.sub.1 -31.sub.n. Referring to FIG. 4A, a constant voltage V.sub.1 is applied to the cable at the station 30.sub.0, and a constant current I.sub.1 is made to flow in the cable from the station 30.sub.n+1 in order to drive the switch rl1 in the branching unit 31.sub.1 in a state in which the node X in the branching unit 31.sub.1 is maintained at the ground level. Thereby, the switch rl1 in the branching unit 31.sub.1 selects the sea ground SE. Next, in order to make the switch rl1 in the branching unit 31.sub.2 select the sea ground in a state where the node X in the branching unit 31.sub.2 is maintained at the ground level, as shown in FIG. 4B, a constant voltage V.sub.2 is applied to the cable at the station 30.sub.0, and a constant current I.sub.2 is made to flow in the cable from the station 30.sub.n+1. Then, as shown in FIG. 4C, a constant voltage V.sub.3 is applied to the cable at the station 30.sub.0, and a constant current I.sub.3 is made to flow in the cable from the station 30.sub.n+1 in order to drive the switch rl1 in the branching unit 31.sub.3 in a state in which the node X in the branching unit 31.sub.3 is maintained at the ground level. Thereby, the switch rl1 in the branching unit 31.sub.3 selects the sea ground SE. In the same manner as described above, the other switches rl1 are sequentially driven to select the sea ground.
In order to drive the switches rl1 in the branching units so that the terminals a and b are connected to each other, the nodes X in the branching units 31.sub.n -31.sub.1 are sequentially set to the ground level in this order.
As described above, it is necessary to sequentially set the nodes X in the branching units 31.sub.1 -31.sub.n to the ground level in order to drive the relays rl1 provided therein. Hence, it is necessary for the currents I.sub.1 -I.sub.n to have different quantities in order to drive only one of the relays rl1 at one time. In practice, it is required that the differences among the quantities of the currents I.sub.1 -I.sub.n be large enough to cope with deterioration in the relay coils with age or the like. Further, it is troublesome to sequentially set the nodes X in the branching units 31.sub.1 -31.sub.n one by one.
Furthermore, the above-mentioned conventional power feed path switching circuit has the following disadvantage. Referring to FIG. 5, a fault has occurred and the branching unit 31.sub.3 is grounded. In this case, the overall branching unit 31.sub.3 is fixed at the ground level. The current I.sub.1 is needed to drive the relay switch rl1 in the branching unit 31.sub.1. However, the branching unit 31.sub.3 is fixed at the ground level, and hence the node X in the branching unit 31.sub.1 is at a potential a with the voltage V.sub.1 applied to the cable at the station 30.sub.0. In this case, the hot-switching phenomenon takes place, and the relay switch rl1 in the branching unit 31.sub.1 may be damaged. Similarly, the current I.sub.2 is needed to drive the relay switch rl1 in the branching unit 31.sub.2. However, the node X in the branching unit 31.sub.2 is at a potential b with the voltage V.sub.2 applied to the cable at the station 30.sub.0. Hence, the hot-switching phenomenon takes place, and the relay switch rl1 in the branching unit 31.sub.2 may be damaged. A similar problem arising from the fault in the branching unit 31.sub.3 will occur in the branching units.